1. Field of the Invention
The present invention relates to a four-cycle OHC engine in which a valve operation system is housed in a valve operation chamber formed between an engine main body and a head cover joined to the engine main body. The valve operation system includes a valve-operating cam that is rotatably supported on a support shaft fixedly supported at its opposite ends in an upper part of the engine main body, and is connected cooperatively to an intake valve and an exhaust valve. A timing transmitting means is housed in a housing passage provided in the engine main body so that the upper part of the housing passage communicates with the valve operation chamber, the timing transmitting means including a driven wheel formed integrally with the valve-operating cam and being disposed between the valve operation system and a crankshaft. Lubricating oil is supplied to the valve operation chamber. In particular, the present invention relates to an improvement in the lubrication structure between the support shaft and the valve-operating cam and driven wheel.
2. Description of the Prior Art
Conventionally, a four-cycle OHC engine is already known in, for example, Japanese Patent Application Laid-open No. 8-177416, wherein lubrication between a support shaft and a valve-operating cam and driven wheel is carried out by employing either a forced lubrication system in which oil is fed under pressure by means of an oil pump to oil passages provided in an engine main body and the support shaft by casting-in or drilling, or a splash lubrication system in which oil is splashed and fills a valve operation chamber and is guided between the support shaft and the valve-operating cam and driven wheel by channels, etc. formed in the wall of an engine main body.
When increasing the rotational speed of an engine in order to enhance the output, it is necessary to supply a larger amount of oil to a lubrication area between the support shaft and the valve-operating cam and driven wheel in order to suppress the generation of heat due to the valve-operating cam and the driven wheel rotating at a higher speed. Employing the above-mentioned forced lubrication system in this case can meet the requirement by increasing the amount discharged from the oil pump, but since it is necessary to machine the engine main body and the support shaft to form the oil passages, the number of machining steps increases and, moreover, the increase in the capacity of the oil pump, etc. inevitably increases the cost.
On the other hand, employing the splash lubrication system can lessen the number of components and the number of machining steps, thereby reducing the increase in cost, but it is difficult to supply a sufficient amount of oil to the lubrication area between the support shaft and the valve-operating cam and driven wheel for reducing the generation of heat due to the valve-operating cam and driven wheel rotating at a higher speed.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to provide a lubrication structure in a four-cycle OHC engine that can supply a sufficient amount of oil to the lubrication area between a support shaft and a valve-operating cam and driven wheel while employing a splash lubrication system which is a low cost system.
In order to accomplish the above-mentioned object, in accordance with an aspect of the present invention, there is proposed a four-cycle OHC engine that includes a valve operation system which is housed in a valve operation chamber formed between an engine main body and a head cover joined to the engine main body and which is operatively connected to an intake valve an exhaust valve. The valve operation system includes a support shaft fixedly supported via its opposite ends in an upper part of the engine main body and a valve-operating cam that is rotatably supported on the support shaft. A timing transmitting means disposed between the valve operation system and a crankshaft and housed in a housing passage, is provided in the engine main body so that the upper part of the housing passage communicates with the valve operation chamber, the timing transmitting means including a driven wheel formed integrally with the valve-operating cam. Lubricating oil is supplied to the valve operation chamber, and an oil intake passage the upper end of which opens upward on the base of the valve operation chamber and the lower end of which is closed, is provided between the upper part of the engine main body and one end of the support shaft, wherein provided on the outside of the lower part of the support shaft is a flat surface for forming an oil passage between the flat surface and the valve-operating cam and driven wheel. One end of the oil passage communicates with the oil intake passage, and the other end of the oil passage opens downward and communicates with the housing passage.
In accordance with the arrangement, together with the use of a splash lubrication system in which oil that has been splashed in and fills the valve operation chamber falls down within the valve operation chamber and is guided to the oil intake passage by free fall, the oil is further guided from the oil intake passage to one end of the oil passage that is formed between the outside of the lower part of the support shaft and the valve-operating cam and driven wheel. The oil can further flow from the other end of the oil passage toward the housing passage and return to the lower part of the engine main body. The oil passage is formed by providing the flat surface on the outside of the lower part of the support shaft and, while suppressing any increase in the machining cost by simplifying the machining of the support shaft, setting the flow areas of the oil intake passage and the oil passage to be comparatively large allows a sufficient amount of oil to be supplied to the lubrication area between the support shaft and the valve-operating cam and driven wheel, thereby reducing the generation of heat due to rotation at higher speeds.
The above-mentioned object, other objects, characteristics and advantages of the present invention will become apparent from an explanation of a preferred embodiment that will be described in detail below by reference to the appended drawings.